1. Field of the Invention
This invention relates to a storage system with a flash memory module using flash EEPROMs as memory elements, and more particularly to a storage system constituting a silicon disk unit used as an external memory device for a computer system.
2. Description of the Related Art
Flash EEPROMs (electrically erasable and programmable read-only memories) are rewritable nonvolatile memory elements that can be erased over the entire chip or in blocks. There are EEPROMs known to be similar to flash EEPROMs. EEPROMs, which are electrically erasable, differ from flash EEPROMs in that the former are erased in bytes.
Unlike EEPROMs, flash EEPROMs do not require a data erase driving circuit for each memory cell, which allows a construction of a single transistor per memory cell. They, therefore, provide a high packing density like other semiconductor memory devices such as DRAMs and mask ROMs.
Flash EEPROMs are characterized by being rewritable but has the disadvantage that there is a limit to the number of possible rewrites. Specifically, data writing may grow more difficult as it is used further, and eventually impossible. Data writing and erasing are done by applying a high voltage such as 12 V or 20 V to inject charges into or extract charges from the floating gate. In the data write operation, to equalize the cell potentials after writing, a high voltage is generally applied in small time divisions until a specified potential is reached, while the write verify operation is being executed on each memory cell.
Recently, storage systems have been developed which employ flash EEPROMs for an external memory device, called a silicon disk unit, for a computer system. Those storage systems are broadly divided into two types: one type is such that a host system recognizes write information in file form, and writes data into flash EEPROMs in a manner that distinguishes file management information from the contents of files, and the other type is such that a host system does not particularly recognize write information but uses the storage system as an external storage device like a conventional hard disk drive (HDD).
With those types, flash EEPROMs can be used as external storage devices, but there is still a limit to the number of possible rewrites in flash EEPROMs as noted earlier. This creates the problem that when the host system writes data in the external memory system, rewrites can concentrate on a particular block of the flash EEPROMs (an in-unison erase block unit), which makes it impossible to rewrite, thereby leading to a serious reduction in the service life of the entire system. With the first type, it is possible to alleviate the concentration of rewrites on a particular block by causing the host system to select a file storage location. To do this, however, the host system needs software to manage the number of rewrites, thereby falling short of universality.
Additionally, when the host system accesses the external storage system, address allocation management is necessary which converts logical addresses in the host system into physical addresses in blocks of flash EEPROMs. Conventionally, because the host system must perform the address allocation management, the system without address allocation management software cannot handle such a memory system, thus leading to lack of universality as mentioned above.